1. Field of the Invention
The present invention relates to an image reading apparatus configured to read an image of a document placed on a document positioning plate and to a reading method therefor.
2. Description of the Related Art
FIG. 9 illustrates a configuration of a conventional color image reading apparatus.
A conventional image reading apparatus, such as the one discussed in Japanese Patent Application Laid-Open No. 2000-13564, reads an image of a document 96 by causing a reading unit 91 to scan the document 96 in parallel with a document positioning glass plate 94. The reading unit 91 includes a linear image sensor. A timing belt 93, which transmits power from a stepping motor 92 serving as a scanning drive source, is fixed to the reading unit 91.
Forward/reverse rotation of the stepping motor 92 enables the reading unit 91 to reciprocatingly scan an area of the document positioning glass plate 94. When reading an image of the document 96, the image reading apparatus drives the stepping motor 92 at a driving speed corresponding to a given resolution to cause the reading unit 91 to perform movement scan.
Generally, the minimum value of an exposure time needed to obtain information representing one line of the document 96 in reading an image of the document 96 is determined according to both the sensitivity of the image sensor and an amount of light received by the image sensor. The maximum value of a movement speed of the reading unit 91 can be determined according to the minimum value of the exposure time needed to obtain such information. When reading an image at low resolution, the reading unit 91 can set a movement speed at a high value. When reading an image at high resolution, it is necessary that the movement speed of the reading unit 91 is low. Actually, the upper limit of the movement speed is subject to various constrains, such as a time in which an electric signal is processed.
A stepping motor, which is useful for easy position control, is used to drive movement scan of the reading unit 91 of the image reading apparatus, and is required to rotate at a speed corresponding to a reading resolution. In a case where an image reading apparatus is being used in an electric power supply limited environment, e.g., where the image reading apparatus is USB (universal serial bus)-powered, the image reading apparatus may be unable to provide a desired reading speed range using a motor having a narrow speed range. Particularly, it is difficult within a range of a rotational speed of a single motor in a conventional high-resolution image reading apparatus to scan an image at reading speeds respectively corresponding to various resolutions. Accordingly, a speed change mechanism, such as a gear change, can be used. In a case where a driving force provided by a single drive source is transmitted using the speed change mechanism, such as a gear change, a state of gears varies during the gear change. Thus, the conventional image reading apparatus cannot accurately achieve position control according to the number of steps of a stepping motor.